Color electroluminescence display device

ABSTRACT

A part of a capacitor is disposed in the horizontal direction of a luminescent region so as to secure a gap in the horizontal direction of the luminescent region. As the distance between adjoining pixels in the horizontal direction is thus secured, color mixing is unlikely, even when an error is caused in positioning a metal mask when forming luminescent layers by evaporation. Further, a portion of the capacitor or TFTs is also disposed in the vertical direction of the luminescent region. Therefore, enhanced color purity can even be achieved for pixels adjoining one another in the column direction in the delta pixel arrangement.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 10/175,978 filed on Jun. 20, 2002, now U.S. Pat.No. 6,995,517 which is a divisional application of U.S. patentapplication Ser. No. 09/451,454, filed on Nov. 30, 1999 now U.S. Pat.No. 6,433,486 and are herein incorporated by reference in theirentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an active type color EL(electroluminescence) display device in which an electroluminescence(EL) element is driven using a thin film transistor (TFT).

2. Description of Related Art

Practical use of organic EL elements in next generation display devicesis greatly expected, because such displays can eliminate need for a backlight as required in a liquid crystal display device for self-emission,can be optimally made thin, and can have an unlimited viewing angle.

Three methods have commonly been proposed for achieving color display ina display device comprising such an organic EL element.

In the first method, different emissive materials for each of theprimary RGB colors are used in corresponding emissive layers toindividually form discrete color pixels directly emitting respective RGBlight rays. In another method, an emissive layer generates whiteluminescence which is then converted into three primary colors usingcolor filters. A third method is based on conversion of light from ablue emissive layer into three primary colors using color conversionmediums (CCM). As light energy is lost in the second and third methodsabove due to the use of color filters or color conversion mediums, thefirst method is the most effective of these in this respect because adesired light ray is directly emitted.

Meanwhile, to drive an organic EL display device, two types of drivingmethods, a passive type using a passive matrix and an active typeemploying TFTs, are available. The circuit configuration shown in FIG. 1may be used in an active display.

FIG. 1 illustrates a circuit configuration for a single pixel in such adisplay pixel. Each pixel comprises an organic EL element 20, a firstTFT 21 for switching, in which a display signal DATA is applied to adrain and a scan signal SCAN is applied to a gate to switch the TFT onand off, a capacitor 22 which is charged by a display signal DATAapplied when the TFT 21 is on and which holds a charge voltage Vh whenthe TFT 21 is off, a second TFT 23 in which a drain is connected to adrive source of a voltage V_(COM), a source is connected to an anode ofthe organic EL element 20 and a hold voltage Vh is applied to a gatefrom the capacitor 22 to drive the organic EL element 20.

A scan signal SCAN rises to an H level during one horizontal scanningperiod (1 H). When the TFT 21 is switched on, a display signal DATA isapplied to one end of the capacitor 22, which is then charged by avoltage Vh corresponding to the display signal DATA. This voltage Vhremains held in the capacitor 22 for one vertical scanning period (1V)even after the signal SCAN becomes a low level to switch the TFT 21 off.Because the voltage Vh is supplied to the gate of the TFT 23, the ELelement is controlled so as to emit light with a luminance in accordancewith the voltage Vh.

The conventional configuration of such an active type EL display devicefor achieving color display by means of the above-mentioned first methodwill be now described.

FIG. 2 depicts a conceptual plan view showing a configuration of arelated art device, and FIG. 3 is a cross section taken along line C-Cin FIG. 2. Each of the drawings depicts three pixels.

In FIGS. 2 and 3, numeral 50 represents a drain line for supplying adisplay signal DATA, numeral 51 represents a drive source line forsupplying a supply voltage V_(COM), and numeral 52 represents a gateline for supplying a scan signal SCAN. Further, numerals 53, 54, and 55designate features corresponding the first TFT 21, the capacitor 22, andthe second TFT 23 in FIG. 1, respectively, and numeral 56 designates ananode of the EL element 20 which constitutes a pixel electrode. Asshown, discrete anodes 56 are separately formed for each pixel on aplanarization insulating film 60. A hole-transport layer 61, an emissivelayer 62, an electron-transport layer 63, and a cathode 64 aresequentially laminated on the discrete anode 56, thereby forming an ELelement. Holes injected from the anodes 56 and electrons injected fromthe cathodes 64 are recombined inside the emissive layer 62, which emitslight in the direction of the transparent anodes toward outside, asshown by arrows in FIG. 3. Here, discrete hole-transport layers 61,discrete emissive layers 62 and discrete electron-transport layers 63having substantially the same shape as the discrete anodes 56 areprovided for respective pixels. Emissive materials which are differentfor each RGB are used in the corresponding emissive layers 62, andtherefore light rays having respective RGB colors are emitted fromrespective EL elements. The cathode 64, which applies a common voltageto each pixel, extends over the pixels. Partitions 68 are interposedbetween adjoining emissive layers 62. Further, numerals 65, 66, and 67designate a transparent glass substrate, a gate insulating film, and aninterlayer insulating film, respectively.

However, the arrangement of the first TFT 53, the capacitor 54, thesecond TFT 55, and the anode 56 of the related examples do not takesufficient consideration of integration efficiency and therefore a morehighly-integrated configuration is in demand.

Further, the color display device generally adopts a stripe arrangementas shown in FIG. 4A or a delta arrangement as shown in FIG. 4C as anarrangement for three primary colors of RGB. At the same time, it isnecessary to use different luminescent materials for each of RGB suchthat discrete EL elements can directly emit light rays of respective RGBcolors. Therefore, if the stripe arrangement shown in FIG. 4A isadopted, for example, a metal mask 70 shown in FIG. 4B may be used toform the luminescent layers as follows. First, a luminescent layer for Ris formed by evaporating only an R color luminescent material onto thehole transport layer. Then, the metal mask 70 is displaced by a distancecorresponding to one pixel in the horizontal direction to form aluminescent layer for G by evaporating only a G color luminescentmaterials on the hole transport layer. Finally, the metal mask 70 isfurther displaced by one pixel in the horizontal direction to form aluminescent layer for B by evaporating only a B color luminescentmaterial. In the case of the delta arrangement shown in FIG. 4C, theluminescent layers can be similarly formed using the metal mask shown inFIG. 4 D.

The above mentioned layer forming methods are disadvantageous in that,if dimensions of the metal mask or positioning at the time of metal maskdisplacement is not accurate, color purity is lowered because colors aremixed in adjoining pixels. Therefore, if a further highly integratedconfiguration is desired, a problem of positioning accuracy in the metalmask may occur.

SUMMARY OF THE INVENTION

The present invention provides a color display device suitable for ahighly integrated configurations.

In accordance with one aspect of the present invention, the ratio of thelength in the horizontal direction to the length in the verticaldirection with respect to the luminescent region in each pixel is setsmaller than the ratio of the pixel pitch in the horizontal direction tothe pixel pitch in the vertical direction with respect to a plurality ofpixels. This configuration provides pixel space in the horizontaldirection, and the color mixture between adjoining pixels which causesdeterioration of color purity can be prevented, even when accuracy ofdisplacing a metal mask for forming a pixel electrode is low.

In accordance with another aspect of the present invention, a capacitoris disposed in a region which adjoins the luminescent region of a pixelin the horizontal direction. Therefore, a highly integratedconfiguration can be achieved while offering space for positioning inthe horizontal direction of a pixel.

In accordance with still another aspect of the present invention, acapacitor is disposed in a region which adjoins the luminescent regionof a pixel in the horizontal and vertical directions. Therefore, ahighly integrated configuration can be achieved while offering space forpositioning in the horizontal and vertical directions of a pixel.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects of the invention will be explained in thedescription below, in connection with the accompanying drawings, inwhich:

FIG. 1 is a view showing a circuit configuration of an active type colorEL display device.

FIG. 2 is a plan view showing a configuration of a conventional color ELdisplay device;

FIG. 3 is cross section showing a configuration of a conventional colorEL display device;

FIGS. 4A, 4B, 4C and 4D are views for explaining color arrangements usedin a color EL display device;

FIG. 5 is a plan view illustrating a first embodiment of the presentinvention;

FIG. 6 is a cross section illustrating the first embodiment of thepresent invention;

FIG. 7 is a plan view illustrating a second embodiment of the presentinvention; and

FIG. 8 is a cross section illustrating the second embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described infurther detail with reference to the accompanying drawings.

Referring to FIG. 5, there is depicted in plan view a color EL displaydevice according to one embodiment of the present invention. It shouldbe noted in FIG. 5 that, for simplicity of explanation and ease ofunderstanding, an organic layer and a cathode of an organic EL elementare not shown and a basic configuration corresponding to three RGBpixels is shown. FIG. 6 is a cross section taken along a line A-A inFIG. 5. This embodiment will exemplify the configuration in which RGBpixels for color display are arranged in a stripe arrangement as shownin FIG. 4A.

A driving circuit for each pixel in this embodiment is the same as thatin FIG. 1, and the configuration of the device differs from the exampleshown in FIGS. 2 and 3 in the pattern arrangement and the cross section.

Referring to FIGS. 5 and 6, the device comprises a data line 1 made ofaluminum for supplying a display signal DATA, a power supply line 2 madeof aluminum for supplying a power from a drive source COM, and a gateline made of chrome for supplying a scan signal SCAN. Each pixel of theEL display further comprises a first TFT 4 corresponding to the firstTFT 21 in FIG. 1, a capacitor 5 corresponding to the capacitor 22 inFIG. 1, a second TFT 6 corresponding to the second TFT 23 in FIG. 1, andan anode (a first electrode) 7 of the EL element 20 comprising ITO andconstituting an pixel electrode. In FIG. 5, regions enclosed by dotlines are formed by chromium, regions enclosed by chain lines are formedby ITO, and regions enclosed by solid lines other than the data line 1and the power supply line 2 are formed using a polysilicon thin film.

The second TFT 6 is formed as follows. First, a gate electrode 9 isformed on a transparent glass substrate 8, and a gate insulating film 10is formed thereupon. Then, a polysilicon thin film 11 formed on the gateinsulating film 10 is covered with an interlayer insulating film 12, onwhich the data line 1 and the power supply line 2 are formed. Aplanarization insulating film 13 is further formed thereon and the anode7 comprising ITO is finally formed on the planarization insulating film13. Then, the drain region of the polysilicon thin film 11 is broughtinto contact with the power supply line 2 while the source region of thepolysilicon thin film 11 is brought into contact with the anode 7.

The configuration of the first TFT 4 is substantially the same as thatof the second TFT 6, with the notable exception that the drain region ofthe first TFT 4 is connected to the data line 1, and not to the powersupply line 2. Further, the capacitor 5 connected to the first TFT 4comprises a chromium electrode and a polysilicon thin film having a gateinsulating film interposed therebetween.

The discrete anodes 7 are formed on the planarization insulating film 13corresponding to respective pixels, and a hole-transport layer 14 isformed thereon so as to cover the entire pixels. Then, discrete emissivelayers 15 are formed for each pixel, on which an electron-transportlayer 16 and a cathode 17 are laminated in this order to completeformation of an EL element. Holes injected from the anode 7 andelectrons injected from the cathode 17 are recombined inside theemissive layer 15, which emits light in the direction of the transparentanode toward outside, as indicated by arrows in FIG. 6. The discreteemissive layers 15 are formed for respective pixels to havesubstantially the same shape as the discrete anodes 7, and differentemissive materials are used for each of the RGB colors. Thus, each ELelement emits one type of RGB light.

Materials of, for example, MTDATA, Alq₃, and MgIn alloy may be used forthe hole-transport layer 14, the electron-transport layer 16, and thecathode 17, respectively. Further, for example, Alq containing DCM typeas dopant is used for the emissive layer 15 for R, Alq containingquinacridon as dopant is used for the emissive layer 15 for G, and DPVBicontaining distyrylarylene or Perylene as dopant is used in the emissivelayer 15 for B.

In this embodiment, as shown in FIG. 5, the second TFT 6 and part of thecapacitor 5 are disposed in the horizontal direction with regard to thepanel of the anode 7 which constitutes a pixel electrode, in otherwords, between the anode 7 of one pixel and another pixel, in particularthe anode thereof, which adjoins in the horizontal direction and emits acolor different from the one pixel. This configuration allows the pixelelectrode to extend further in the vertical direction compared to therelated art example shown in FIGS. 2 and 3. Further, the luminescentregion of a pixel has substantially the same shape as the pixelelectrode since the pixel electrode 7 and the emissive layer 15 havesubstantially the same shape. Accordingly, assuming that the dimensionsof the luminescent region in the horizontal and vertical direction areEH and EV, respectively, and that the dimensions of the pixel pitch inthe horizontal and vertical direction are PH and PV, respectively,EH/EV<PH/PV is found.

Thus, when forming respective RGB emissive layers by displacing metalmasks for each RGB with one another in the horizontal direction withrespect to the panel, as shown by an arrow in FIG. 4B, a room for suchdisplacement of the metal masks in the horizontal direction is enlargedcompared to the conventional configuration. As a result, the likelihoodof colors in adjoining emissive layers being mixed can be reduced, evenwhen emissive layers using different materials for each colors areformed with the same positioning accuracy as that in the relatedexamples. Here, the first TFT 4, in place of the second TFT 6, may bedisposed in the horizontal direction of the anode 7.

However, during the process for forming the luminescent layers byevaporating the luminescent materials, a so-called “diffusion”phenomenon is caused in which the luminescent materials are evaporatedonto regions other than the regions directly under the openings of themetal masks 70 and 71. Due to such a diffusion phenomenon or inaccuratedimensions of the metal mask itself, the colors are adversely mixed inadjoining pixels to deteriorate color purity. In particular, in the caseof the delta arrangement in which adjoining pixels, both in the columnand row directions, have colors which are different from one another,this disadvantage is further notable.

In this embodiment, a portion of the capacitor 5 is disposed in thehorizontal direction of the luminescent region and continuously extendsin the vertical direction of the luminescent region. Thus, at least partof the capacitor 5 or a thin film transistor is disposed in the verticaldirection of the luminescent layer of each pixel, such that space isprovided between pixels in the vertical direction. Therefore, it ispossible to achieve high quality display with preferable color purity,even when the accuracy of metal mask positioning is low.

FIG. 7 depicts, in plan view, the second embodiment of the presentinvention, and FIG. 8 is a cross section taken along line B-B of FIG. 7.It is to be noted that in these drawings the same elements as shown inFIGS. 5 and 6 are designated by the same reference numerals, and thatthe second embodiment differs from the first embodiment only in thepattern arrangement. Further in FIG. 7, as in FIG. 5, the organic layerand the cathode are eliminated for simplicity of explanation and ease ofunderstanding.

In FIGS. 7 and 8, numeral 4 denotes a first TFT corresponding to thefirst TFT 21 of FIG. 1, numeral 5 denotes a capacitor corresponding tothe capacitor 22 of FIG. 1, numeral 6 denotes a second TFT correspondingthe second TFT 23 of FIG. 1, and numeral 7 denotes an anode comprisingITO and constituting a pixel electrode of the EL element 20. As clearlyshown in FIG. 8, the capacitor 5 is constituted by a chromium electrode500 and a polysilicon thin film 501 having a gate insulating film 10therebetween.

In this embodiment, as shown in FIG. 7, the capacitor 5 is disposed nextto the anode 7 which constitutes a pixel electrode in the horizontaldirection with respect to the panel. Namely, as in the first embodiment,the capacitor 5 is necessarily disposed between the anode 7 of one pixeland the anode of another pixel which adjoins in the horizontal directionand emits a color different from the one pixel. This configurationallows the pixel electrode to extend further in the vertical directionthan in the related art example shown in FIGS. 2 and 3. Further, as inthe foregoing embodiment, when the dimensions of the luminescent regionin the horizontal and vertical directions are EH and EV, respectively,and the dimensions of the pixel pitch in the horizontal and verticaldirections are PH and PV, respectively, the relationship EH/EV<PH/PV isfound true.

Thus, when forming respective RGB emissive layers such that metal masksfor each RGB are displaced with each other in the horizontal directionwith respect to the panel, as shown by an arrow in FIG. 6B, a room forsuch displacement of the metal masks in the horizontal direction isenlarged compared to the conventional configuration. As a result, thepossibility that colors in the adjoining emissive layers may be mixedcan be diminished even when the respective emissive layers are formedwith the same positioning accuracy as that in the related examples.

According to this embodiment, the capacitor 5 is disposed in thehorizontal direction of the luminescent region, and both of the firstand the second TFTs 4 and 6 are disposed in the vertical direction ofthe luminescent region. Thus, a gap between the luminescent regions inadjoining pixels in the horizontal direction can be secured by thecapacitor 5 while a gap between the luminescent regions in adjoiningpixels in the horizontal direction can be secured by the TFTs 4 and 6,thereby further preventing color blur due to diffusion, as in the firstembodiment.

It is to be noted that features other than the capacitor 5 may, ofcourse, be disposed to secure the gaps between the luminescent regions.However, a capacitor is most preferable for the following reason.Namely, since the capacity is proportional to the area in a capacitor,by increasing the capacity of a capacitor so as to securely hold avoltage Vh in accordance with a display signal Data, the area of thecapacitor can be increased to secure necessary gaps. Accordingly, it ismost efficient to adjust the gaps by disposing the capacitor 5.

According to the present invention, color mixture in adjoining pixelswhich causes deterioration of color purity can be prevented in an activetype color EL display device, thereby maintaining a preferable colorpurity even in a highly detailed display.

Specifically, by setting the ratio of the horizontal length to thevertical length of the luminescent region of each pixel to be smallerthat the ratio of the horizontal pixel pitch to the vertical pixelpitch, space is provided in each pixel in the horizontal direction suchthat a high quality display can be produced, even when accuracy formetal mask displacement is low.

Further, by disposing at least part of a capacitor or a thin filmtransistor in the horizontal direction of the luminescent region of eachpixel, space is provided in each pixel in the horizontal direction suchthat highly detailed display can be achieved even when metal maskdisplacement accuracy is low.

Although the present invention is also applicable to a delta pixelarrangement, the above-mentioned effects are especially significant whenemployed with a stripe arrangement.

1. An active matrix color electroluminescence display device comprising: an electroluminescence element having a luminescent layer between an anode and a cathode; a first gate line and a second gate line which are parallel to each other; a drain line and a power supply line which intersect the gate line; a first thin film transistor connected to the first gate line; a second thin film transistor which drives the electroluminescence element; and a capacitor for holding a voltage to be supplied to the second thin film transistor, wherein the first thin film transistor, the second thin film transistor, and an emissive region are disposed in a region surrounded by the first gate line, the second gate line, the drain line, and the power supply line; the emissive region adjoins the second gate line and one of the drain line and the power supply line and does not adjoin the first gate line and the other one of the drain line and the power supply line; and the second thin film transistor adjoins the second gate line and the other one of the drain line and the power supply line.
 2. A color electroluminescence display device according to claim 1, wherein the first thin, film transistor adjoins the first gate line and the other one of the drain line and the power supply line.
 3. A color electroluminescence display device according to claim 2, wherein the capacitor is connected to a source of the first thin film transistor and to a gate of the second thin film transistor and adjoins the other one of the drain line and the power supply line.
 4. A color electroluminescence display device according to claim 3, wherein the capacitor is disposed between the emissive region and the first thin film transistor and between the emissive region and the other one of the drain line and the power supply line.
 5. A color electroluminescence display device according to claim 1, wherein the capacitor is connected to a source of the first thin film transistor and to a gate of the second thin film transistor and adjoins the other one of the drain line and the power supply line.
 6. A color electroluminescence display device according to claim 1, wherein the capacitor is disposed between the emissive region and the first thin film transistor and between the emissive region and the other one of the drain line and the power supply line.
 7. A color electroluminescence display device according to claim 1, wherein the one of the drain line and the power supply line is the drain line.
 8. A color electroluminescence display device according to claim 1, wherein the emissive regions are formed for same color, and colors of the emissive regions which are adjacent along a direction of extension of the gate line differ from each other.
 9. A color electroluminescence display device according to claim 1, wherein the luminescent layer is evaporated using a mask.
 10. A color electroluminescence display device according to claim 1, wherein the emissive regions are arranged in a matrix form in a stripe arrangement which emissive regions of a same color are placed adjacent to each other along a column direction. 